A study of DDR-type zeolite crystals and membranes

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Abstract

A zeolite membrane consists of a homogenous layer of intergrown zeolite crystals
synthesized on the surface of a ceramic support. DDR zeolites consist of three types of
window connected silica oxide cages that build up the rhombohedral DDR structure and
have pore apertures of 0.36 x 0.44nm. Membranes (formed by a hydrothermal synthesis
process) are gaining an important place in chemical technology as they are able to
selectively control the permeation rate of chemical species that pass through it. The DDR
membrane has been shown to separate CO2 (g) from CH4 (g).
The objective of this research was to acquire a better understanding of the properties of
DDR zeolites in combination with ceramic membranes. Therefore investigations were
initially carried out on the manufacture of ceramic membranes using Alcoa powder in
conjunction with research on the DDR-type zeolite crystals, as the hydrothermal synthesis
period of the DDR crystals is a lengthy 25 days. The aim of this investigation was to
acquire a better understanding of DDR zeolites through optimization of the experimental
procedure. This thesis investigated the coating of the DDR-type zeolite on to a ceramic
membrane which has been accomplished only once before.
The manufacture of ceramic membranes was investigated using a cheaper source of a-alumina
powder (Alcoa powder). The first treatment of pH flotation was optimized at a
loading mass of 200g Alcoa powder suspended in a pH2 solution of HNO3 with APMA as
the dispersing agent. As a result of this acid treatment, SEM analysis showed that the fine
particles remained in suspension and the heavier particles settled.
Secondly, a novel fractionation process using the heavy particles that had settled in the pH
flotation showed optimum separation results at linear velocities of 5ml/min and 15ml/min
using a 75g loading mass of Alcoa powder. Thereafter, centrifugal casting of the fractions
was carried out to produce asymmetrical tubular green casts. Finally, a programme of
sintering allowed for strengthening and transformation of the green cast into a ceramic
support.
To synthesize a DDR membrane, the crystals firstly had to be made and used as seeds to
accelerate the growth of a DDR membrane. It was found that when the water concentration
was decreased from 11240 moles to 7838 moles of water, homogenous crystals (1.4m) of
well defined morphology were obtained. During the hydrothermal synthesis of a DDR
membrane various factors were investigated.
Results showed that a support refluxed in HN03 (aq) had improved zeolite attachment
when compared to the pre-treatment of sonification. A seeding mass of 0.008g and a ten
fold increase to 0.08g did not show a difference in the amount of coverage of the support
with DDR crystals. When the seeding techniques of immersion and centrifugation were
used, the same homogenous, but inadequate seed coverage was seen. Irrespective of the
synthesis parameters investigated, a gel was consecutively produced on the support after
hydrothermal synthesis. Only when the hydrothermal synthesis period was increased from
48 hours to 96 hours, some crystallization occurred.
This investigation on the manufacture of ceramic support was partly successful in that a
cheaper Alcoa ceramic membrane was reached, although repeatability was poor. SEM and
XRD analysis confirmed the size and purity of DDR crystals after using an optimized
synthetic procedure. Information from this thesis lays the foundation for the successful
synthesis of a DDR membrane, as it has provided valuable information to direct the future
research on this topic.